Rectification of liquefied coke oven gas portion by contact between liquefied and revaporized portions thereof



April 8, 19-69 JAKQB 3,436,925

RECTIEICATION OF LIQUEFIED COKE OVEN GAS PORTION BY CONTACT BETWEEN LIQUEFIED AND REVAPORIZED PORTIONS THEREOF Filed Sept. 14, 1966 INVBNTOR FRITZ JAKOB A TORNEY 3,436,925 RECTIFICATION F LIQUEFIED COKE OVEN GAS PORTION BY CONTACT BETWEEN LIQUEFIED AND REVAPORIZED PORTIONS THEREOF Fritz Jakob, Wolfratshausen, Germany, assignor to Linde Aktiengesellschaft, Wiesbaden, Germany Filed Sept. 14, 1966, Ser. No. 579,338 Claims priority, applicatioil G8e7rmany, Sept. 21, 1965, 5 6

Int. Cl. Fis 3/02 U.S. Cl. 62-27 10 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a rectification process for the separation of a mixture of components having a Wide boiling point distribution, and in particular to such a process conducted at low temperatures for the production of cryogenic gases.

1n rectification processes for the separation of components having widely different boiling points, the mixture to be separated is fed in the liquid phase to the head of a rectification column, and this liquid, acting as reflux, is then stripped of its low boiling components. The vapor flowing upwardly against the liquid feed is produced by boiling the bottoms in a reboiler or the like. This system is feasible because even if the mixture has a substantial proportion of high-boiling component, the gaseous phase in equilibrium with the boiling mixture, i.e., the overhead product from the column, is so poor in high-boiling components that there are no significant losses of high boiling components.

In these known processes, however, a substantial thermal inefliciency exists because of the large AT between still and overhead product. This problem is especially severe in the production of cryogenic gases where the available temperature level of refrigeration is very important. Thus, for example, in a low temperature rectification process, the sump liquidalmost pure highboiling component-must be evaporated at a temperature ranging considerably above the temperature at 'which the mixture fed to the head of the column is condensed. This means that refrigeration must be available at a substantially lower temperature level than it can be recovered during the vaporization of the sump liquid. Therefore, in such cryogenic processes, it is necessary to provide a heat pump system in order to accomplish the transfer of refrigeration values.

An object of this invention, therefore, is to provide an improved process for the rectification of a mixture having a wide boiling point distribution.

A more particular object is to provide an improved process for the rectification of a mixture, wherein said mixture is fed as liquid to the top of the rectification column.

A further object includes the aforementioned objects as applied to low temperature processes wherein refrigeration energy is consequential, and in particular to the production of enriched hydrogen gas from coke oven gas, and also to the production of ethylene.

atent O $436325 Patented Apr. 8, 1969 Upon further study of the specification and claims, other objects and advantages of the present invention will become apparent.

For the attainment of the above objects, a portion of the liquid mixture, heretofore used only as reflux, is vaporized and the resultant vapor is passed into the bottom portion of a rectification column. The remaining unvaporized liquid is fed as reflux to the head of this same rectification column.

Thus, not the entire liquid mixture is introduced to the head of the column as reflux liquid, but only a portion thereof. The other portion is vaporized and introduced into the rectification column at the foot thereof in vapor form. Vapor rising within the column is, therefore, produced by vaporizing a part of the feed mixture, as compared to prior art processes wherein all vapor was obtained by the vaporization of sump liquid.

This improved process is of particular lbenefit in the case of low temperature rectification processes, because the vapor is not obtained at the relatively high vaporizing temperature of the sump, but at the sliding vaporization temperature of the mixture, which temperature gradient is, on the average, substantially lower than that of the sump. Thereby, the process of the invention-as compared to the conventional processes-requires considerably less expensive means for transferring refrigeration values. This fact, together with the use of smaller heating surfaces, made possible by the more favorable heat exchange conditions, results in a substantial decrease in the plant investment and operating costs. Corresponding considerations apply in connection with a rectification conducted at higher temperatures.

This invention is particularly beneficial for the treatment of feed streams wherein about 30 to 80, of preferably 50 to 70 mol percent of the feed has an average boiling point of about at least 15 (1., preferably at least 30 C. higher than 70 to 20, preferably 50 to 30 mol percent of the feed. Furthermore, it is to be noted that the process of this invention is even more important for cryogenic processes wherein at least 30 mol percent, preferably at least 50 mol percent of the feed has a boiling point below about 40 0, preferably below about C.

With respect to the portion of the liquid feed that is vaporized and passed as vapor to the bottom of the rectification column, it is preferred that at least 20 percent, more preferably at least 30 percent of the liquid feed be so utilized.

For example, from a liquid feed with a content of 60% CH (boiling point 16l C.) and 32% CO (boiling point 191.5 0.), 6% N and 2% other substances-condensed by cooling of coke oven gasthere is vaporized an amount of about 40%, and there is used about 60% as reflux. The boiling point of the bottom product is then ca. 169 C. and of the head product ca. -192 C., the extent of separation of boiling points being 23 C.

Because of the boiling point distribution of the feed components, the liquid, which is in equilibrium with the introduced vapor at the foot of the rectification column and is collected in the sump, is so rich in high-boiling component that it represents an acceptable product, un less, of course, it is necessary to produce a highly pure product.

In case several separate streams of varying analyses are to be combined to form the liquid, that portion of the mixture which is to be vaporized and introduced into the rectification column is advantageously formed from the stream which is richest in the high-boiling mixture component, or in the high-boiling group of mixture components, in order to obtain the high-boiling component which is the purest possible for this particular case.

According to one embodiment of this invention, when producing methane from a liquid mixture as obtained during the cooling of coke oven gas, one portion of this mixture can be vaporized in heat exchange with coke oven gas. It is furthermore possible, when producing methane from coke oven gas, to adrnix the low-boiling components exiting at the head of the rectification column directly to the cold residual gas.

The attached drawing is a schematic representation of a preferred embodiment of this invention, illustrating that section of a coke oven gas separating plant important for the production of methane from coke oven gas, and for the purpose of producing a hydrogen-nitrogen mixture for the ammonia synthesis.

Referring to the drawing in greater detail, the precooled coke oven gas is fed to the plant via conduit 1 and is further cooled in heat exchangers 2 and 3. During this procedure, a part of the coke oven gas condenses, namely practically all of CH CO, and N This condensate is separated in the liquid-vapor separator 4 from the remaining coke oven gas and fed, in two portions via different paths, to the rectification column 5. One portion, about 80 to 30%, is fed directly via conduit 6 to rectification column 5 where it is employed as reflux. The other portion is conducted, by conduit 7, to heat exchanger 3, evaporated therein, and subsequently introduced into column 5 at the foot thereof. The liquid, at the vapor feed point, is very substantially enriched in methane, owing to the favorable boiling relationships of CH on the one hand, and CO+N2, on the other hand; consequently, this liquid can be withdrawn directly as a product from the bottom of column. 5 through conduit 8.

If there is present, for example, a 60% methane fraction, a portion of which is vaporized and introduced into the rectification column, the liquid which is in equilbrium with this vapor is enriched with methane to approximately 95%. Thus, the process of this invention can, in such a case, yield methane ot a 95 purity.

The product is then vaporized in the heat exchanger 2 at a relatively high temperature (the boiling temperature of the moderately pure methane); thus, it transfers its refrigeration values at this relatively high temperature. The condensate separated in the liquid-vapor separator, however, must be formed at a lower temperature in the heat exchanger 3. In order to achieve this objective, one portion of the condensate is additionally vaporized in the heat exchanger 3. Since the condensate still contains substantial proportions of CO and N its median boiling temperature is considerably lower than the boiling temperature of the sump liquid, or of the product. Consequently, the vaporization of a portion of the condensate in the heat exchanger 3, yields refrigeration ot a lower temperature.

The residual gas withdrawn from the head of column 5, consisting of CO, N and a small proportion of methane, is admixed, in front of the heat exchanger 3, with residual gas in conduit 9 coming from the nitrogen washing stage of the coke oven gas separating plant. The resultant mixture of these residual gases is warmed in the heat exchangers 3 and 2. The hydrogen-nitrogen mixture coming from the nitrogen iwashing stage is likewise conducted through these heat exchangers, via conduit 10.

A typical analysis of coke oven gas in conduit 1 which can be treated by this invention is Component: Vol. percent range H 50-57 N 6l2 CO 5-7 CH 22-25 C l-L, 12 C H 0.7-2 C H 30.6 CGHG 0. -1 CO 2-5 Aside from its use in the precedingly described embodiment, the teachings of this invention can also be particularly advantageously employed in the production of ethylene where the boiling relationships between ethylene, on the one hand, and methane, on the other hand, are likewise favorable for the production of a moderately pure product.

In this connection, ethylene and methane have normal boiling points of ll9 C. and l61 C., respectively, thereby giving a mixture of such liquefied gases a boiling point separation of 42 C. As an example of a specific process, a feed mixture comprising 45% ethylene and 55% methane is treated so as to vaporize 54% thereof, thereof, with the residual 46% being employed as reflux. The resultant bottoms product contains ethylene, and in the resultant overhead gas, ethylene is lost to an extent of only about 12%.

Numerous other process streams to be rectified can likewise be treated, as those skilled in the art most assuredly will recognize; and it is intended for this patent to secure protection in all such instances.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Consequently, such changes and modifications are prop erly, equitably, and intended to be, within the full range of equivalence of the following claims.

What is claimed is:

1. In the rectification of a mixture of components having a wide boiling point distribution which comprises feeding said mixture in the liquid phase as reflux to the top part of a rectification column, the improvement comprising:

passing the gaseous feed mixture in serial flow through successive first and second heat exchangers, the sec 0nd of which is at a lower temperature to liquefy a part of the feed mixture, removing all of the liquid and vapor portions of the feed mixture to a separation zone to separate the liquid phase mixture, feeding a liquid portion of 30-80% of said liquid phase rnixture as reflux to the head of said rectification column, vaporizing the remaining other portion of liquid phase mixture in countercurrent fiow exclusively through said second heat exchanger externally of said rectification column in heat exchange with the components of the feed mixture, and passing said vaporized portion into the bottom part of said column in direct contact with sump liquid therein, said liquid portion and vaporized portion being the only two feed streams to said rectification column, whereby a smaller temperature difierential is maintained between overhead vapor and sump liquid product leaving said rectification column than would be the case if sump liquid were vaporized in a reboiler and passing sump liquid product in countercurrent and exclusive heat exchange through said first heat exchanger.

2. A process as defined by claim 1 wherein said mixture in the liquid phase is derived from different streams having dilferent analyses, and wherein said other portion of said liquid mixture which is vaporized is formed from the stream having the highest content of high-boiling component.

3. A process as gaseous stream is coke oven gas.

4. A process as defined by claim 1 further comprising the step of passing said overhead vapor from the rectification column also in indirect heat exchange relationship with said gaseous stream.

5. A process as defined by claim 1, wherein 30-80 mol percent of said liquid phase mixture has an average boiling point of about at least 15 C. higher than 70-20 mol percent of said liquid phase mixture.

described by claim 1 wherein said 6. A process as defined by claim 1, wherein at least 30 mol percent of said liquid phase mixture has a boiling point below about 40 C.

7. A process as defined by claim 1, wvherein 30-80 mol percent of said liquid phase mixture has an average boiling point of about at least C. higher than 70-20 mol percent of said liquid phase mixture, and at least mol percent of said liquid phase mixture has a boiling point below about C.

8. A process as defined by claim 1, wherein -70 mol percent of said liquid phase mixture has an average boiling point of about at least 30 C. higher than 50-30 mol percent of said liquid phase mixture.

9. A process as defined by claim 1 wherein 50-70 mol percent of said liquid phase mixture has an average boiling point of at least 30 C. higher than 50-30 mol percent of said liquid phase mixture; and at least 50 mol percent of said liquid phase mixture has a boiling point below about C.

10. A process as defined by claim 9 wherein said mixture in the liquid phase is derived from difierent streams having different analyses, and wherein said other portion of said liquid mixture which is vaporized is formed from the stream having the highest content of high-boiling component or the highest average boiling temperature.

References Cited UNITED STATES PATENTS 2,559,132 7/1951 Roberts 6229 XR 2,664,719 1/1954 Rice et a1. 6229 XR 2,743,590 5/1956 Grunberg 6228 2,997,854 8/1961 Schilling et a1. 6229 XR 3,209,548 10/1965 Grunberg et al 6213 XR 3,264,831 8/1966 Jakob 6213 2,071,763 2/ 1937 Pollitzer 6212 2,817,216 12/1957 Etienne.

NORMAN YUDKOFF, Primary Examiner.

V. W. PRETKA, Assistant Examiner.

US. Cl. X.R. 6223 

